August 27, 1998
GENERAL RELEASE SPECIFICATION
External Ratiometric Reference
If readings are to be ratiometric to some external source, the reference measure-
ment point can be connected to any one of the channel selected pins from PB4
through PB7. In this case the reference reading can be taken by setting the MUX
bit in the AMUX which connects channel selection bus to the pin connected to the
external reference source.
Table 15-7. Ratiometric Voltage Reading Errors
Accuracy Improvements Possible
Error Source
In Hardware
In Software
Change in
Reference Voltage
Compare unknown with recent
measurement from reference
Not required for ratiometric
Change in Magnitude of
Ramp Current Source
Compare unknown with recent
measurement from reference
Not required for ratiometric
Not adjustable
Non-Linearity of Ramp
Current Source vs. Voltage
Calibration and storage of voltages
at 1/4, 1/2, 3/4 and FS
Change in Magnitude of
Ramp Capacitor
Compare unknown with recent
measurement from reference
Not required for ratiometric
Not required for ratiometric
Not required for ratiometric
Use faster conversion times
Frequency Shift in Internal
Low-Power Oscillator
Compare unknown with recent
measurement from reference
Frequency Shift in
External Oscillator
Compare unknown with recent
measurement from reference
Sampling Capacitor
Leakage
Compare unknown with recent
measurement from reference
Internal Voltage Divider
Ratio
Compare unknown with recent
measurement from reference
Not required for ratiometric
Close decoupling at
Average multiple readings on both the
reference and the unknown voltage
Noise internal to MCU
Noise external to MCU
V
and V pins and
DD SS
reduce supply source impedance
Close decoupling of power supply, low
source impedances, good board layout,
use of multi-layer board
Average multiple readings on both the
reference and the unknown voltage
15.6 VOLTAGE COMPARATOR FEATURES
The internal comparator can also be used as simple voltage comparator.
Voltage Comparator
Voltage comparator can be used as a simple comparator if its charge current
source and discharge device are disabled by clearing the ISEN bit in the ACR. If
ISEN bit is set the internal ramp discharge device connected to CAP may become
active and try to pulldown any voltage source that may be connected to that pin.
Also, since voltage comparator is always connected to two of the Port B I/O pins,
these pins should be configured as inputs and have their software programmable
pulldowns disabled. The required setup to use voltage comparator as a simple
comparator are shown in Table 15-8.
MC68HC05SB7
REV 2.1
ANALOG SUBSYSTEM
MOTOROLA
15-29
FREESCALE [ Freescale ]
